arm: dts: santvend: Make the ldb node disabled by default

If the ldb node is enabled in device tree but parameters of connected LVDS panel are not defined, graphic subsystem will fail to start during boot. As a result, there will be no video output even on connected HDMI display. Disable the ldb node out of box in device tree, and enable it back in post installation step of fng-install via device tree overlay if LVDS output is enabled in config.xml.